It has been argued that canopy trees in tropical rainforests harbor species-rich ant assemblages; however how ants partition the space on trees has not been ade- quately elucidated. Therefore we investigated within-tree distributions of nest sites and foraging areas of individual ant colonies on canopy trees in a tropical lowland rainforest in Southeast Asia. The species diversity and colony abun- dance of ants were both significantly greater in crowns than on trunks. The concentration of ant species and colonies in the tree crown seemed to be associated with greater vari- ation in nest cavity type in the crown compared to the trunk. For ants nesting on canopy trees the numbers of colonies and species were both higher for ants foraging only during the daytime than for those foraging at night. Similarly for ants foraging on canopy trees both values were higher for ants foraging only during the daytime than for those foraging at night. For most ant colonies nesting on canopy trees foraging areas were limited to nearby nests and within the same type of microhabitat (within-tree position). All ants foraging on canopy trees in the daytime nested on canopy trees whereas some ants foraging on the canopy trees at night nested on the ground. These results suggest that spatial partitioning by ant assemblages on canopy trees in tropical rainforests is affected by micro- environmental heterogeneity generated by three-dimen- sional structures (e.g. trees epiphytes lianas and aerial soils) in the crowns of canopy trees. Furthermore ant diversity appears to be enriched by both temporal (diel) and fine-scale spatial partitioning of foraging activity.